High-pressure hot pressing of silicon powder with sintering aids and subsequent high temperature sintering of pressed silicon bodies are known in the literature.
Pellet pressing of powders is well known in metallurgical and ceramic process industries. All such processes utilize some binder or additive to effect compaction and pressing. In some instances sintering aids are purposely added in the pellet process. The binders/additives/aids leave a residue of organic or inorganic nature during subsequent use or other operations. In addition, pressed bodies are sintered at high temperatures to provide pellet strength. Pellet pressing is also utilized in pharmaceutical industries to manufacture medicinal tablets. Inert additives and binders, innocuous to patients, are utilized to convert drug chemical powders to compacted tablets.
Ultra fine silicon is a by-product of the Fluid Bed process to manufacture high purity electronic grade polysilicon. In this process silicon is deposited by thermal decomposition of silane (SiH4) gas on granules of silicon seed particles. The granules grow in size from an initial seed size of about 0.2 mm (millimeter) to about 1 mm in diameter. The granules are utilized in silicon melting and crystal growth applications.
The Fluid Bed process, however, also results in a large quantity of ultra fine silicon dust. This is tapped out of the reactor and remains as a process waste. This powder is of high purity, but cannot be recycled or used in silicon melting applications.
There are also other sources of less pure silicon powder, such as reaction residues from preparation of organochlorosilanes or chlorosilanes from the reaction of elemental silicon with chlorinated hydrocarbons or hydrogen chloride. The powder is used as a feed in alternate but less demanding industries such as for steel hardening, addition to aluminum melts, manufacture of silicon nitride, and so forth. For such applications the powder is agglomerated with a binding agent to form granules of 250–500 microns. The binders are typically organic materials such as starch, and lignin. Other agglomeration methods include microwave heating of the powder (particles of less than about 100 μm) to 1200°–1500° C.
Where it is necessary to stabilize silicon dust and powder and make them into a more stable form for transportation and disposal (deactivation of silicon) the silicon dust is milled in an aqueous solution of pH>5 to form colloidal silica. This helps to agglomerate the dust.
High-pressure hot pressing of silicon powders are described in the art, such as in The Effects of Processing Conditions on the Density and Microstructure of Hot-Pressed Si Powder, by C. J. Santana and K. S. Jones, J. Materials Sci. 31 (18), 4985–4990 (1996); and High Pressure Hot-Pressing of Si Powders, by K. Takatori, M. Shimada and M. Koizumi, J. Jap. Soc. Powder Metal. 28 (1) 15–19 (1981).
In one such application silicon powder was hot pressed into polycrystalline wafers 1.5″ diameter using various process conditions, typically hot pressing at 1300° C./2000 psi in hydrogen gas ambience. Densification typically proceeds with increasing temperature and applied pressure. There are also several studies of sintering silicon compacts at high temperatures, ranging from 1250° C. to close to the melting point of Si (1412° C.), in an inert atmosphere. Silicon sintering with addition of sintering aids such as Boron, or retardants such as Tin, is described in the art. There is no way demonstrated in the art to obtain a dry silicon pellet from silicon powder without added binder or added heat.